Answers and discussions on the inspection of end caps in the Safety Technical Inspection Regulations for Fixed Pressure Vessels TSG 21-2016

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As a fundamental component of pressure vessels, pressure vessel heads are widely used. Question: Regarding Section 6.1.2.1 of the TSG 21-2016 Regulations on Safety Technical Supervision for Fixed Pressure Vessels, which requires that welded end caps undergo manufacturing supervision inspections, under normal circumstances, when a pressure vessel manufacturer purchases welded finished end caps from another supplier, the end cap manufacturer will provide a (Certification of Manufacturing Supervision for Special Equipment) ； It has been found that the pressure vessel manufacturer carried out lap welding on externally manufactured heads; the head manufacturer provided a Certificate of Supervision and Inspection for the outsourced head manufacturing. Does this Certificate of Supervision and Inspection meet the requirements of Section 6.1.2.1 of TSG 21-2016? Is it equivalent to the (Special Equipment Manufacturing Supervision and Inspection Certificate)? Answer: The head forming work can be outsourced, but welding cannot. Processes such as welding must be carried out in facilities that have the appropriate manufacturing permits. Headings that are shipped separately and have welds or require heat treatment after shaping need to be subject to inspection. Time: 2023-08-09 Discussion Discussant 1.1: I think the answer is not clear. In my opinion, according to TSG 21-2016, for container manufacturers that carry out lap welding, end caps that are manufactured externally do not require a supervision and inspection certificate at all. Discussant 2.1: There is indeed a lot of controversy. I’m not sure whether it’s a matter of our understanding – does the rule that welding cannot be outsourced mean that supervision is required whenever outsourcing is involved, or is it an absolute ban? If it’s an absolute ban, then does that mean that flanged end caps cannot be purchased from outside (or are end caps excluded)? Discussant 1.2: Personally, I believe that according to the provisions of TSG 21-2016, integral factory-assembled heads with seams (welded by the head manufacturer) require supervision and inspection. The container manufacturer performs the welding itself; only the shaping is outsourced (heat treatment is not included). Since it is not a fully sealed container when it leaves the factory, inspection is not required. Discussant 3.1: There are many issues with the head seal joint test plates not meeting the standards. Discussant 2.2: In practice, it is done this way. Another issue is that if the head to be outsourced requires inspection, should the base metal specimens and the weld seam specimens of that head be inspected by the special inspection agency corresponding to the head manufacturer, or by the special inspection agency corresponding to the manufacturing factory? Discussant 4.1: It should be the local special inspection agency that inspects the base metal specimens and weld seam specimens, depending on who performs the welding of the head’s weld seams. Discussant 1.3: This is a complex issue; some of the base metal test plates also need to undergo heat treatment along with the equipment. Discussant 2.3: It also involves whether final annealing is needed later on. Comment 4.2: If the equipment requires post-weld heat treatment, we generally require that the size of the test piece be sufficient for at least two tests; half of it is tested after the head is formed, and the other half is tested after the equipment undergoes post-weld heat treatment. Discussant 1.4: You can do it that way, but if the heat-treated test plate turns out to be unqualified, whose responsibility will it be? Discussant 4.3: The specimens manufactured after normal head forming passed the inspections, while those after the final heat treatment of the equipment failed them; it’s not easy to hold the head manufacturer accountable. Discussant 2.4: The end cap factory performs maximum and minimum simulations. Discussant 5.1: It shouldn’t be the end-cap manufacturer; it should be the steel plate manufacturer who handles it, meaning requirements need to be specified when ordering the steel plates. Discussant 4.4: Even with maximum-minimum simulations, differences may still exist, after all, the heat treatment equipment used by the two companies is different; simulated heat treatment is carried out in enclosed small furnaces, while the heat treatment furnaces of the actual equipment sometimes fail to achieve the same results as those small furnaces. Discussant 2.5: The steel plate factory is the base material, while the head factory is involved in welding as well. Discussant 4.5: Steel plate manufacturers only carry out simulated heat treatment for the maximum and minimum values of the materials, and they do not provide any guarantees for the welded specimens. Discussant 5.2: Welding often relates to strength issues after thermoforming, which can be resolved by replacing the material; this is generally not a problem with conventional sheets. Another discussion: Discussant 1.1: Any welding work that is outsourced must be subject to supervision and inspection. The maximum and minimum values are not determined by the steel plate manufacturers; in fact, it is often difficult to control the quality of end cap manufacturing. What about expansion joints used in pressure vessels, if it’s only the end caps that are considered? The General Administration’s response is rather vague. Discussant 2.1: Sometimes the manufacturing plants end up in a passive position. Discussant 1.2: What if I outsource the head fabrication but carry out stress relief at the head factory? I have a different opinion: because of the thinning of the head, you may need several pieces for your samples. I don’t agree with using samples from the base material; I think it’s more representative to take samples by making holes in the head. Discussant 2.2: Indeed, the test plates haven’t been pressed or anything like that... Discussant 1.3: I went to the head manufacturing factory a few days ago and asked a question: You said that the final pressing temperature should be no lower than AR3, so how is the temperature measured in that case? Take a few points: how is the mold temperature controlled during the first molding and the tenth molding, as the number of molding cycles increases? Discussant 2.3: The test plate cannot represent the actual heating process of the head; it’s not clear how the new standard will address this, as it probably doesn’t include any information on this topic. Discussant 1.4: No, after going through many processes of renewing and obtaining certificates, these are all controversial issues. New additions in the revised version of GB/T25198-2023: 1. Requirements for pressure vessel heads made of zirconium materials have been added, along with requirements for pressure vessel heads made of aluminum, copper, titanium, nickel and their alloys as well as composite sheets ； 2. New terms have been added: forming heating temperature (the highest temperature reached by the head blank or semi-finished product during the heating process) and final pressing temperature (the temperature at which plastic deformation of the head blank or semi-finished product comes to an end) ； 3. A tapered head structure without flanges has been added ； 4. Regarding the marking on the end caps, it is required that the manufacturer indicate the thickness of the material used ； 5. Requirements for the manufacturing environment have been added: The manufacturing environment for stainless steel, non-ferrous metal, and composite plate end caps must be clean and meet the requirements of relevant standards. 6. Added risk assessment and control provisions for head manufacturing ； 7. The requirements for panel assembly have been revised ； 8. The specifications regarding the surface color of welds and heat-affected zones in titanium and titanium alloy heads have been revised, and specifications for the surface color of welds and heat-affected zones in zirconium heads have been added ； 9. Add requirements for the heating process: During heating, the temperature of the workpiece should be measured and recorded ； Increase the requirements for controlling the heating temperature of zirconium and zirconium composite plates: Zirconium end caps as well as zirconium composite plate end caps should be formed by heating, with the forming temperature generally being no lower than 500℃ ； Increase the recommended temperature for warm forming of austenitic stainless steel heads: Warm forming is suitable for austenitic stainless steel heads, with a recommended forming temperature of 120°C to 250°C℃ ； 10. Control requirements for increasing composite plate manholes should use the inner perimeter or inner diameter as the alignment basis ； The diameter range has been expanded to 10,000 mm, and the tolerance requirements for the outer perimeter and inner diameter have been modified ； 11. Additional requirements have been added for the control and measurement of the ferrite content in austenitic stainless steel heads designed for temperatures below -40°C and those that have undergone strain strengthening treatment. 12. Requirements regarding heat treatment documentation as well as automatic recording of heat treatment temperatures have been added: The heat treatment furnaces used for these heads must meet the specified standards, and they should be equipped with temperature measuring instruments that can automatically record the temperature curve, thereby enabling the creation of a graph showing the relationship between heat treatment time and the wall temperature of the workpiece. 13. Revised the values of deformation rate used to determine whether annealing is necessary after cold forming of oval, disc, and conical end caps made of aluminum-magnesium alloys and aluminum-magnesium-silicon alloys, titanium and titanium alloys, and brass (except H96) ； 14. The impact of the deformation rate resulting from sheet leveling on head manufacturing has been taken into account: if the steel sheet used undergoes leveling, the deformation rate caused by this process must be included, and the effect of such deformation shall be considered by the head manufacturing unit. 15. Requirements for product test pieces/welding base metal heat treatment test pieces have been added: 16. Taking into account the advancements in non-destructive testing technologies, new non-destructive testing methods have been added and the relevant requirements have been revised ； 17. The requirements for factory delivery documents were clarified ； 18. Requirements for product markings have been added: When the head leaves the factory, a product marking must be applied in a visible location on it (usually at the end of the head, about 10 mm away from that end). Methods for marking can include labels, nameplates, inkjet printing, as well as markings that do not cause corrosion to the head. Any special requirements regarding marking methods and locations must be specified in the technical specifications for ordering ； 19. Appendix B, which provides informational data, includes tables with the total depth, internal surface area, volume, and mass of HHA spherical heads ； 20. Appendix H, which is of informational nature, adds content on the measurement of ferrite in stainless steel heads ； 21. Appendix I, which provides informational content, includes the density of zirconium materials ； 22. Appendix J, which is of informational nature, lists the thinning rates for THA/THB and EHA/EHB end caps using common forming methods (cold/heat stamping, cold/heat spinning, and splitting), along with the corresponding values for diameters up to 10,000 mm, and also outlines the commonly used forming methods ； 23. The technical specifications for ordering end caps in Appendix K, which is of informational nature, include additional requirements regarding the welds used to join the end caps, the types of grooves used for welding, requirements for welding test pieces and heat-treated base metal test pieces, as well as requirements for measuring the ferrite content ； 24. Appendix K, which is of informational nature, adds the types and codes for head grooves.